1. Field of the Invention
The present invention relates generally to a method of manufacturing a semiconductor device and, more particularly, to a semiconductor device manufacturing method for forming wells which are indispensable to a CCD (charge coupled device), a CMOSIC (complementary metal oxide semiconductor integrated circuit) or the like.
2. Description of the Prior Art
FIGS. 1A through 1C are cross-sectional views showing examples of respective semiconductor devices having wells. FIG. 1A shows an example of a CCD image sensor and FIGS. 1B, 1C show examples of CMOSICs, respectively. FIG. 1C shows the example of CMOSIC in which an isolating n-type well is formed in order to isolate a p-type well and an n-type well when a negative bias voltage is applied to a p-type semiconductor substrate. Throughout FIGS. 1A to 1C, reference symbols a and b depict p-type wells and reference symbol c depicts an n-type well. Generally, it is known that the wells a, b and c are formed by selectively diffusing impurities on a surface portion of the semiconductor substrate.
Since the wells are formed by the diffusion of impurities in the prior art, it takes an extremely long time to form wells. That is, a diffusion time of several 10s of hours calculated in terms of 1100.degree. C. is required. If such long diffusion time is required as described above, then productivity of semiconductor device is naturally lowered, which becomes a factor for hindering semiconductor devices from being produced inexpensively. Also, impurities are diffused in the lateral direction and hence devices cannot be miniaturized like micro-devices. Further, there is the risk that the core tube, wafer holding boat or the like will be deformed by heat. Furthermore, there is the risk such that a metal in a high temperature heater will be diffused into the core tube to contaminate the surface portion of the semiconductor wafer.